SingerX,
at first, here are some basics on Butterworth approximation:
1.) For comparison, you need the n-th order lowpass function (normalization Ω=ω/ωc, ωc=3dB cut-off):
A(jΩ)=ao/[1+d1(jΩ)+d2(jΩ)²+d2(jΩ)³+...+]
2.) The MAGNITUDE function for a Butterworth approximation (n-th order) is
A(Ω)=Ao/SQRT[1+Ω^(2n)]
3.) Now you have to find the MAGNITUDE of the first expression and perform a comparison of the coefficients (d1...dn) ) with the second expression in 2.).
(That´s a nice task!!)
4.) Find the transfer function of the CIRCUIT you have in mind and compare again the coefficients of the corresponding parts in the denominator. This gives you parts values (R, L, C).